Corrupt Spell (Metamagic)

You can transform one of your spells into a thing of evil due to a deal you make with an evil power.

(Champions of Ruin, p. 17)

Originally posted on D&Dtools

• Prerequisite – Any evil alignment,
• Benefit – This feat adds the evil descriptor to one damaging spell you can cast. You must choose the spell at the time you select the feat. If the spell deals damage, half of the damage is unholy damage. For example, a corrupted fireball cast by a 6th-level wizard deals 6d6 points of damage: 3d6 points of fire damage and 3d6 points of unholy damage. Thus, creatures immune to fire still potentially take 3d6 points of damage.
• Special – A character can take this feat multiple times, choosing a different spell each time.